Revert "blkdev: check for valid request queue before issuing flush"

This reverts commit f10d9f61.

We can't have queues without a make_request_fn any more (and the
loop device uses blk-mq these days anyway..).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>